The United States is refusing to give a visa to the Iranian official nominated by Tehran to be its next ambassador to the United Nations.

The decision effectively bars Hamid Aboutalebi from taking up his post at the UN, which is based in New York.

Mr Aboutalebi has been linked to the student group that took US diplomats hostage in 1979. He maintains he only acted as an interpreter and negotiator.

It is understood the US has never before denied a visa for a UN ambassador.

President Barack Obama has come under strong pressure from Congress over Mr Aboutalebi's suspected participation in the Muslim student group that held 52 Americans hostage for 444 days when the group seized the US embassy in Tehran in 1979.

White House spokesman Jay Carney said the United Nations and Iran had been told that a visa would not be issued to Mr Aboutalebi.

"We've communicated with the Iranians on a number of levels and made clear our position on this, and that includes our position that the selection was not viable and our position that we will not be issuing a visa," Mr Carney said.

Neither the White House nor the State Department provided further explanation.

The decision has sparked concerns the dispute will disrupt delicate negotiations between Tehran, Washington and other world powers over Iran's nuclear program.

A spokesman for Iran's mission to the UN has described the decision as "regrettable", saying it is "in contravention of international law".

US law allows the government to bar UN diplomats who are considered national security threats, but Mr Obama's potentially precedent-setting step could open the US to criticism that it is using its position as host nation to improperly exert political influence.

Mr Obama's decision came days after negotiators from Iran, the United States and five other world powers met in Vienna for another round of nuclear talks.

But an Iranian official said he did not expect the dispute to affect the nuclear negotiations.

Any official response would be up to the Iranian Foreign Ministry, but the US decision "will have no impact on our talks with the P5+1," the official told Reuters.

US officials also said they did not expect any impact.

The United Nations said it had no comment at this time on the US decision.